1. Field of the Invention
This invention pertains generally to thermionic cathodes and more particularly to reservoir-type dispenser cathodes which find particular advantageous application in cathode ray tubes that require relatively high current density.
2. Prior Art
The most relevant prior art known to the applicant is a co-inventor's previous patent, U.S. Pat. No. 4,823,044 issued Apr. 18, 1989 for A DISPENSER CATHODE AND METHOD OF MANUFACTURE THEREFORE. That patent discloses a dispenser cathode which employs a novel structure, permitting a significant reduction in cost for a cathode capable of achieving extremely high current densities, such as for use in cathode ray tubes. The structure of that dispenser cathode is conducive to a uniform level of performance throughout the life of the cathode, namely uniformity of current density. The configuration of that prior invention produces a uniform flow of barium from a reservoir enclosed pellet. The barium passes through a pure tungsten enclosing pellet which has a porous configuration. The porous, pure tungsten pellet needs no impregnation because the activating barium is derived entirely from the underlying enclosed pellet. The pure tungsten overlying pellet and the underlying barium source pellet configuration, prevents clogging of pores in the tungsten pellet and also prevents current density changes or patchiness, both instantaneously and over the life of the cathode. The prior art dispenser cathode of U.S. Pat. No. 4,823,044, comprises four separate pieces, namely a pressed and sintered porous tungsten pellet, a pressed pellet made of barium calcium aluminate and tungsten, a punched, pressed reservoir formed of molybdenum, rhenium, a combination molybdenum and rhenium, tantalum, or other refractory metal and a support cylinder in the form of an extrusion or similarly processed structure formed of molybdenum, molybdenum/rhenium or tantalum. The resulting cathode is designed to operate at approximately 850-1,150 degrees centigrade, depending upon the current density objectives. The pellet contained within the reservoir provides a constant low level of barium evaporation to activate the tungsten in the overlying pellet.
The need for a high current density, relatively inexpensive cathode is driven by the demand for higher resolution cathode ray tubes for high definition television (HDTV), automotive displays, computer graphic displays, projection television and avionic applications. These new applications for cathode ray tubes require the employment of cathodes capable of producing higher current densities than those presently obtainable from the triple carbonate oxide cathode. In other than short pulse applications, the triple carbonate oxide cathode system, which has been the industry standard for decades, produces emission densities of less than an ampere per centimeter squared and therefore cannot be used in applications where higher densities are required A cathode system which will meet the market demand for higher resolution must be capable of achieving two design criteria. First, a smaller diameter electron beam bundle which produces a smaller spot size at the viewing surface is required. This smaller, electron beam is produced by using smaller apertures in the beam forming region (BFR) of the electron gun. Secondly, because brightness levels for these high resolution applications must be maintained, the currents of these smaller diameter electron beams must be the same as those of the conventional larger beam diameter systems. To achieve this goal, a cathode system must operate at a higher current density
The characteristic behavior of an oxide cathode is related to the fact that it is essentially a dielectric material and will "charge up". It can only achieve high current densities in Short pulse length applications. Oxide cathodes are also susceptible to poisoning, requiring exacting and lengthy tube processing to obtain the best performance characteristics. The life of oxide cathodes and cathode ray tube guns is relatively short, particularly in applications where the current density is in excess of a few hundred milliAmperes per square centimeter. Because the dielectric nature of an oxide cathode limits the current density, a metal emitter as used in dispenser cathodes must be considered for cathode ray tubes.
The impregnated dispenser cathode, the most typical use of which is in microwave tubes, is made from porous tungsten which is impregnated with barium compounds. When heated, the barium compounds react with the tungsten matrix, allowing the barium to migrate to the surface of the cathode. Throughout its use, the cathode surface is constantly covered with barium and the emitter surface work function drops from 4.5 electron volts to as low as 2.0 electron volts. While the impregnated dispenser cathode is capable of producing high current densities and long lifetime use, it must be operated at about 200 degrees centigrade higher than the oxide cathode. In addition to requiring a higher operating temperature to produce the higher current density, this cathode also requires a longer activation cycle. These two performance characteristics result in excessive evaporation of the barium, which can cause unwanted grid emission and high voltage instability. Because of this and because the conventional impregnated dispenser cathode is more expensive to manufacture than the oxide cathode, the reservoir dispenser cathode was considered superior for use in cathode ray tube applications.
The reservoir cathode was the original type of dispenser cathode. With this design, barium compounds are held in a cavity or reservoir behind a porous disk, such as that disclosed in the aforementioned prior art patent of the applicant, namely U.S. Pat. No. 4,823,044. When heated, the compounds decompose or react with a reducing agent. The barium is then dispensed through the porous disk to the surface. While this novel reservoir cathode is a significant improvement over the previous art in terms of life and cost to manufacture, the porous disk through which the barium is dispensed, such as the tungsten overlying disk described in U.S. Pat. No. 4,823,044, has a porosity which is dependent upon the pressure, temperature and starting materials used in its fabrication. Furthermore, the number, size and location of the "pores" that are produced, such as for example by pressing and sintering pure tungsten, are random and relatively difficult to control. Consequently, the work function and life of each such prior art reservoir dispenser cathode may be somewhat unpredictable and vary from a maximum which may be otherwise attainable by careful control of the size, number and location of the pores through the overlying disk. Consequently, a need exists for improving the aforementioned reservoir dispenser cathode, by utilizing an emitter structure having a porosity which is not random, but which is geometrically controlled in a precise and predictable fashion, thereby permitting optimization of the various advantages previously derived from the invention disclosed in U.S. Pat. No. 4,823,044.
U.S Pat. No. 4,101,800 issued Jul. 18, 1978 relates to controlled porosity dispenser cathodes using metal foil made of a refractory metal and having selected holes made therein.
Another factor in considering a cathode for use in cathode ray tubes is its life expectancy. A long-life cathode is usually one which can operate at a reduced level of heater minimized work function permits lower operating temperature, but it is also highly desirable to have a high thermal efficiency structure which provides a commensurate reduction in heater power to produce the lower operating temperature. Inefficiency in the use of heater power to provide even a reduced operating temperature would be self-defeating. A cathode having high current density, controlled emitter porosity and long life expectancy due to a low operating temperature and an improved thermal efficiency structure, would indeed be desirable.